In the field of semiconductor processing, typically damascene processes are used to form copper-based interconnects. In damascene processes, trenches are etched in low dielectric constant materials and these trenches are then subsequently filled up to create the copper interconnects.
During performance of an etch process unwanted portions of a material layer deposited on a substrate may be removed, thereby forming a desired material layer pattern. In case of a damascene process, trenches and vias are etched in a dielectric material film. Typically a resist pattern is deposited first onto the dielectric layer and the dielectric layer is then selectively etched with respect to the resist pattern thereby, for example, using an anisotropic plasma formed in a reactive ion etching chamber. Therefore, a negative voltage is applied to an electrode which is located below the loaded wafer. In response to the negative voltage the positive ions in the plasma are pulled towards the wafer and are absorbed upon impact with the portions of dielectric film exposed through resist pattern. When absorbed, the ions chemically react with atoms in the dielectric layer under the influence of kinetic energy imparted by the applied electric field to form a volatile compound. In this manner, the plasma etching process selectively removes the exposed portion of a dielectric layer. The endpoint of the etching process is typically determined by over-etching in the Cu structure underneath. This over-etching leads to Cu contamination of the chamber walls of the reaction chamber because Cu is sputtered away and will re-deposit onto these chamber walls. Since Cu is unwanted in the dielectric material of a semiconductor device, the removal of these Cu residues from the chamber walls is extremely important. Since Cu is very difficult to volatilize, a state of the art cleaning process (e.g. Wafer Auto Clean (WAC) processes) is not sufficient because sputtering the Cu deposits will only lead to a re-deposition of the residues onto other parts of the reaction chamber. A chamber cleaning process used to clean the reaction chamber in order to remove most of the copper nowadays often consists of dismantling the chamber and performing a wet etch with citric acid.
FIG. 1 schematically illustrates such a state of the art chamber cleaning procedure used to remove copper residues in e.g. a reactive ion etching chamber. FIG. 1(a) illustrates the plasma chamber 1 with a wafer holder 2, also referred to as wafer chuck, and a wafer 3 located on the wafer chuck 2 and used as a substrate onto which plasma processing is performed. Performance of the plasma process leads to copper residues 4 sticking onto the chamber walls. A prior art WAC cleaning (e.g. using a plasma of O2/SF6 or an O2/Cl2 plasma) may be performed to remove contaminants mainly consisting of organic polymeric residues. However, after performing such a WAC cleaning, the copper residues 4 will still be present on the chamber walls (see FIG. 1(b)). According to prior art methods a wet cleaning procedure using e.g. citric acid is required to remove the copper contamination 4. Such wet etch requires the dismantling of the plasma chamber 1 which is very time consuming (see FIG. 1(c)). Moreover, after each cleaning process, stabilization of the plasma chamber is required (see FIG. 1(d)) before another wafer process can be started.
The above-described cleaning procedure is thus time consuming and cannot be performed after each wafer which implies that Cu contamination is hardly impossible to be avoided during processing, because that would take even more time.